The invention relates to a device for generating a cold plasma in an enclosure under reduced pressure, and relates more particularly to a cathode suitable for generating a high-power cold plasma.
A person skilled in the art is perfectly familiar with the industrial use of cold plasmas for altering the surface of various types of materials, in order to create active species, out of equilibrium, which do not exist in the natural state. The use of plasmas in thermochemical treatment allows accurate control of the concentration of active species, as opposed to conventional methods. Low-pressure cold plasmas further serve to significantly limit the gas consumption, which is advantageous, both economically and ecologically.
Various technical solutions have been proposed for generating a plasma under reduced pressure.
According to one solution, the parts to be treated are directly polarized. A luminescent discharge is created in the near vicinity of the surface, thereby exciting the gas mixture selected. This technique is widely used in case hardening or ion nitriding, but creates heterogeneities of treatment according to the geometry and the arrangement of the parts with regard to one another. In fact, the natural intensification of the plasma in particular zones, such as holes, edges, etc., causes local overheating which unfavourably affects the quality of the treatment. It also appears in this technique that the substrates are subject to intense ion bombardment, which alters their surface.
Another solution appears from the teaching of patent FR 2 702 119 which uses an antenna for injecting a body wave into the enclosure, like a microwave oven. The main drawback of this technology is that antennas are point sources which can rarely inject more than 200 W. It is therefore necessary to increase the number of sources to have a uniform plasma in an industrial enclosure. This causes an increase in the size, costs and risks of failure. This type of antenna only serves to inject a microwave signal (no DC nor pulsed-DC nor RF signals) and only provides its best yield in the particular physical conditions of electron cyclotron resonance.
Another solution appears from the teaching of patent FR 2 766 321 which relates to a gas excitation device comprising a hollow structure forming a waveguide, the structure being connected to a generator, while means circulate the gas through the structure. A plasma is created outside the structure or enclosure, in a hollow tube made from dielectric material. The results obtained are only satisfactory at relatively high pressures for a vacuum technology (higher than 5 mbar), which causes gas flow problems inside the enclosure, hence a treatment heterogeneity.
Another solution appears from the teaching of patent EP 0 872 569 which relates to an ion bombardment nitriding method, in which a plasma is generated in a furnace. A current is conveyed to the metal lattice cathode, which surrounds the articles to be treated, the anode consisting of the furnace walls. The cathodes used are generally ineffective for creating large quantities of active species. The ion bombardment to which the cathodes are subjected causes their heating, which contributes to the heating of the parts, making it impossible to dissociate the temperature of the parts from the generation of the plasma (hence of the active species). A further drawback encountered with this type of cathode is its sputtering due to high voltages (about 500 volts), causing pollution of the substrates.
To remedy these drawbacks, in order to limit the voltage of the cathode, it has been proposed to confine the plasma in holes in order to intensify it and to lower the voltage. One solution of this type appears from the teaching of patent EP 0 881 865, which relates to a device for producing a plurality of low-temperature plasma jets by means of a high frequency power source, using discharges. The device comprises a plurality of individual hollow cathode chambers, each plasma jet being associated with an individual hollow cathode chamber, as a discharge enclosure. Among the drawbacks generated with this solution, a power imbalance may be observed between the holes, and the total power is always deliberately maintained at a level often lower than 1 kW, due to the overheating of the assembly resulting from a very intense ion bombardment in the holes.
It is the object of the invention to remedy these drawbacks simply, safely and effectively.